1. Field of the Invention
The invention relates to a structure of a monolithic array-type light emitting device having a plurality of light emitting parts in one device (chip) and a process for producing the same, and more particularly to a monolithic array-type light emitting device suitable particularly as a light source for printers.
2. Prior Art
Printers of a xerography system utilizing light emitting diodes (LEDs) have been put to practical use.
In this system, LEDs satisfying emission wavelength and emission intensity required based on the light receiving sensitivity of a photoreceptor should be selected. LED arrays using GaAsP as a main material of p/n junction in the light emitting device and LED arrays using GaAlAs as a main material of p/n junction in the light emitting device have been adopted for practical use.
For emission output which is one of important characteristics of LED, in order to provide products having the highest possible emission output, an attempt has been made to efficiently take out light from one direction using the so-called xe2x80x9cBragg-typexe2x80x9d multilayered reflection film. The semiconductor multilayered reflection film comprises a plurality of layers of high-refractive index xcex/4n films and low-refractive index xcex/4n films wherein xcex represents emission wavelength of LED and n represents refractive index. In this case, light, which has been emitted from the active layer and directed to the substrate side, is reflected from the multilayered reflection film and exits from the top surface of the device. This can improve the light takeout efficiency.
The provision of the multilayered reflection film, however, poses a new problem of increased resistance of the device. Specifically, in the AlGaAs material used in the multilayered reflection film, the band offset of the valence band at the interface of junction between the high-refractive index film and the low-refractive index film is so large that the injection of holes is not easy, leading to significantly increased resistance of the device. The increased resistance of the device leads to increased forward voltage which adversely affects characteristics of the device and results, for example, in increased power consumption and generation of heat.
Further, in conventional AlGaAs-based LEDs, when AlGaAs materials are adopted in the multilayered reflection film provided between the substrate and the active layer, in terms of the reflectance, it is considered that the light takeout efficiency increases with increasing the refractive index difference and the number of pairs. This had led to a tendency toward the adoption of GaAs in the thin layer on the high-refractive index side while adopting AlyGa1xe2x88x92yAs, wherein 0 less than yxe2x89xa61, in the thin layer on the low-refractive index side. The use of GaAs as the high-refractive index film for the purpose of providing higher reflectance, however, further increases the band offset at the interface of each hetero junction constituting the multilayered reflection film, resulting in significantly increased resistance of the device.
The wavelength of the LED array for LED printers is determined by materials used in pn junction in LED. When GaAsP or AlGaAs is used as the material for the pn junction, the wavelength of LED is in many cases 700 to 800 nm. This is because a wavelength region, which provides high emission output, is selected according to materials. On the other hand, in the case of LED printers, the wavelength sensitivity of a photoreceptor, which receives light emitted from LED, is also an important parameter, and the emission wavelength is restricted by a combination of the light emitting device with the photoreceptor. For LED printers, there is an increasing demand for higher speed and higher resolution, and LED arrays with higher output are required. At the present time, however, the above-described materials do not always satisfy the demand at emission efficiencies determined by the materials per se.
Accordingly, high emission output is desired in AlGaAs-based LED arrays for LED printers because the emission output has direct influence on printing speed. To satisfy this requirement, the realization of LED arrays having high output while providing enhanced light takeout efficiency through the adoption of the multilayered reflection film, double hetero (DH) structure or the like is necessary.
Accordingly, it is an object of the invention to solve the above problems of the prior art and to provide a light emitting device of an LED array which, while utilizing the advantage of improved light takeout efficiency realized by providing a Bragg-type multilayered reflection film, can suppress an increase in resistance of the device attributable to the provision of the multilayered reflection film and, in its turn, an increase in forward voltage, and can realize high emission efficiency and high output on the whole.
The above object can be attained by an LED array-type light emitting device wherein, in an AlGaAs-based LED array device having a Bragg-type multilayered reflection film comprising a plurality of reflection layers, the aluminum (Al) composition ratio of AlGaAs constituting the layers of the multilayered reflection film is specified to make reflection efficiency higher than the case where a high-refractive index GaAs layer is adopted as one reflection layer in the multilayered reflection film, and to suppress an increase in resistance of the device attributable to the provision of the Bragg-type multilayered reflection film and, in its turn, to suppress an increase in forward voltage, thereby realizing high output.
Specifically, according to the first feature of the invention, a monolithic array-type light emitting device comprising a plurality of light emitting parts in one device, wherein: each of said light emitting parts comprises a light emitting diode having a laminate structure comprising an n-type GaAs substrate and, epitaxially grown on the n-type GaAs substrate in the following order, an n-type GaAs buffer layer, an n-type laminated reflection film formed of layer pairs each comprising AlGaAs layers different from each other in Al (aluminum) composition ratio, an n-type AlGaAs lower cladding layer, a p-type or undoped AlGaAs active layer, a p-type AlGaAs upper cladding layer, and a p-type GaAs contact layer; and each of the layer pairs constituting the laminated reflection film is a multilayered film of an AlX1Ga1xe2x88x92X1As layer and an AlX2Ga1xe2x88x92X2As layer where X1 and X2 each represent Al composition ratio, wherein hetero junction has been formed between the AlX1Ga1xe2x88x92X1As layer and the AlX2Ga1xe2x88x92X2As layer, wherein the Al composition ratio is X1 less than X2 and the refractive index is n1 greater than n2 where n1 represents the refractive index of the AlX1Ga1xe2x88x92X1As layer and n2 represents the refractive index of the AlX2Ga1xe2x88x92X2As layer, wherein the AlX1Ga1xe2x88x92X1As layer and the AlX2Ga1xe2x88x92X2As layer satisfy X1xe2x89xa7X and X2xe2x89xa7X where X1 and X2 are as defined above and X represents Al composition ratio in AlXGa1xe2x88x92XAs constituting the active layer, and wherein the AlX1Ga1xe2x88x92X1As layer satisfies EgX1xe2x89xa7Excex wherein EgX1 represents the band gap energy of the AlX1Ga1xe2x88x92X1As layer and Excex represents emission wavelength energy.
In the light emitting device according to the first feature of the invention, the density of the light emitting parts is preferably not less than 240 dpi (dots per inch).
In any one of the above light emitting devices, preferably, the size of a light emitting region in each LED constituting the light emitting parts is 50xc3x9750 xcexcm or less.
In any one of the above light emitting devices, preferably, an electrode contact layer having a size of not more than 10xc3x9750 xcexcm is provided on each of the light emitting regions, an ohmic contact part is provided on a part of the top of each of the contact layers, and anodes for respective wirings are drawn respectively from the ohmic contact parts.
In any one of the above light emitting devices, preferably, the anodes for wirings provided in the p-type contact layer are arrayed in a given direction on one side of the light emitting parts perpendicular to the direction of the row of each of the light emitting parts, or alternatively are provided alternately on one side of the light emitting part and on the other side of the light emitting part in a direction perpendicular to the direction of the row of each of the light emitting parts.
In the light emitting device according to the invention, individual light emitting parts are arranged and arrayed in one row or two or more rows.
According to the second feature of the invention, a process for producing a monolithic array-type light emitting device having a plurality of light emitting parts in one device comprises the steps of: forming, by MOVPE, an epitaxial wafer for a light emitting diode having a structure comprising an n-type GaAs substrate and, epitaxially grown on the n-type GaAs substrate in the following order, an n-type GaAs buffer layer, an n-type laminated reflection film formed of layer pairs each comprising AlGaAs layers different from each other in Al (aluminum) composition ratio, an n-type AlGaAs lower cladding layer, a p-type or undoped AlGaAs active layer, a p-type AlGaAs upper cladding layer, and a p-type GaAs contact layer; and forming the plurality of the light emitting parts, wherein each of the layer pairs constituting the laminated reflection film is formed as a multilayered film of an AlX1Ga1xe2x88x92X1As layer and an AlX2Ga1xe2x88x92X2As layer where X1 and X2 each represent Al composition ratio, wherein hetero junction has been formed between the AlX1Ga1xe2x88x92X1As layer and the AlX2Ga1xe2x88x92X2As layer, wherein the Al composition ratio is X1 less than X2 and the refractive index is n1 greater than n2 where n1 represents the refractive index of the AlX1Ga1xe2x88x92X1As layer and n2 represents the refractive index of the AlX2Ga1xe2x88x92X2As layer, wherein the AlX1Ga1xe2x88x92X1As layer and the AlX2Ga1xe2x88x92X2As layer satisfy X1xe2x89xa7X and X2xe2x89xa7X where X1 and X2 are as defined above and X represents Al composition ratio in AlXGa1xe2x88x92XAs constituting the active layer, and wherein the AlX1Ga1xe2x88x92X1As layer satisfies EgX1xe2x89xa7Excex wherein EgX1 represents the band gap energy of the AlX1Ga1xe2x88x92X1As layer and Excex represents emission wavelength energy.
In the invention, a distributed Bragg reflection (DBR) film having an AlGaAs multilayered structure is used for effectively taking out light directed to the substrate side.
Here one of the films constituting the Bragg-type laminated reflection film, i.e., a high-refractive index film, is formed of AlX1Ga1xe2x88x92X1As (refractive index n1), while the other film constituting the Bragg-type laminated reflection film, i.e., a low-refractive index film, is formed of AlX2Ga1xe2x88x92X2As (refractive index n2). In this laminated reflection film, the relationship in refractive index n1 greater than n2 is a requirement for increasing the reflection and is necessary for increasing the refractive index difference to enhance the reflectance.
When two reflection layers constituting the semiconductor multilayered film are AlX1Ga1xe2x88x92X1As and AlX2Ga1xe2x88x92X2As (X1 less than X2), the compositions of the respective layers constituting the multilayered film are selected so that (1) the refractive index difference is increased, (2) the absorption for the object wavelength can be minimized, and (3) an increase in resistance of the device can be minimized. The laminated reflection film may be, for example, nxe2x88x92Al0.25Ga0.75As/Al0.85Ga0.15As laminated reflection film.
A structure satisfying these requirements is advantageous in that the forward voltage at the band offset point is lower than that of the Bragg-type laminated reflection film, in which GaAs has been adopted in one of the layers constituting the laminated reflection film, and that the emission wavelength is less likely to be absorbed in the layers constituting the laminated reflection film, whereby high emission efficiency can be realized.
In the invention, two AlGaAs layers, which are different from each other in aluminum composition ratio, constituting each layer pair in the laminated reflection film (aluminum composition ratio X1 less than X2, refractive index n1 greater than n2) are constructed so that the two layers satisfy X1xe2x89xa7X and X2xe2x89xa7X where X represents aluminum composition ratio in AlXGa1xe2x88x92XAs constituting the active layer, and the AlX1Ga1xe2x88x92X1As layer satisfies EgX1xe2x89xa7Excex wherein EgX1 represents the band gap energy of the AlX1Ga1xe2x88x92X1As layer and Excex represents emission wavelength energy. This construction can realize higher reflection efficiency than that the structure wherein a high-refractive GaAs layer is adopted as one of the layers constituting each layer pair in the laminated reflection film.